Brakes that use compressed air instead of fluid to stop heavy vehicles.
An engine-driven compressor fills tanks with air. When you press the pedal, that air pushes the brakes on. Air brakes are really three systems: service, parking and emergency.
On the test
Questions ask what air brakes use to work, which parts belong to the system, and why a separate air brakes test exists.
Appears in 72 test questions
See 47 questions on the Air Brakes testExample: Before you pull out of the yard, you wait for the air pressure to build because your truck has air brakes.
Also called: air brake, air brake system
The engine-driven pump that fills the air tanks with compressed air.
The engine turns the compressor through gears or a belt. Some compressors have their own oil supply; if yours does, check the oil level before driving. At operating rpm a dual system should build from 85 to 100 psi within 45 seconds.
On the test
Questions ask what the compressor does, how it is driven, what to check on it, and what holds air in the tanks if it fails.
Appears in 11 test questions
See 9 questions on the Air Brakes testExample: During the pre-trip inspection you check the compressor drive belt for wear and tightness.
The control that starts and stops the compressor.
When tank pressure reaches the cut-out level, the governor stops the compressor. When pressure drops to the cut-in level, it starts the compressor again.
Differs by state: The nominal cut-out value differs by handbook edition. Forty-one state handbooks, including Texas, New York and California, say around 125 psi. Minnesota and Pennsylvania say around 120-140 psi and never print 125. Cut-in is around 100 psi in every handbook. See cut-out pressure.
Example: At fast idle you watch the gauge needle stop rising; that is the governor cutting out the compressor.
Also called: air compressor governor
The tank pressure at which the governor stops the compressor.
The gauge needle stops rising at cut-out. Your handbook gives a usual value, but the real setting is the manufacturer's specification for your truck.
Differs by state: Forty-one state handbooks, including Texas, New York and California, give the governor cut-out as around 125 psi. Minnesota and Pennsylvania print around 120-140 psi instead and never print 125. Most handbooks also give a 120-140 psi range for the pre-trip air check, so both figures can appear in one book. The value is a manufacturer setting, not a state law.
Does not mean: Cut-out cocks are shut-off valves on trailer air lines; they have nothing to do with the governor's cut-out pressure.
Example: You run the engine at fast idle until the needle stops climbing, which shows you the cut-out pressure.
Also called: cut-out level, cut out
The tank pressure at which the governor restarts the compressor.
Pressure falls as you use the brakes. When it reaches cut-in, usually around 100 psi, the compressor starts again and the gauge needle begins to rise.
On the test
Questions ask when the compressor starts pumping again and how you lower tank pressure to see the cut-in during the check.
Example: With the engine idling you fan the brake pedal until the needle starts rising again at cut-in.
Also called: cut-in level, cut in
A reservoir that stores compressed air for the brake system.
The tanks hold enough air to brake several times even if the compressor stops. Water and compressor oil collect at the bottom and can freeze, so the tanks must be drained.
Example: At the end of your shift you open each tank's drain valve and let the water and oil run out.
Also called: air tanks, air reservoir, storage tank
The valve under each air tank that lets out water and oil.
Manual valves open with a quarter turn or a pull cable, and you drain them yourself each day. Automatic valves expel the moisture on their own.
Example: You pull the cable on each tank to drain it, then make sure the valve is shut before you drive.
Diagram coming
Also called: tank drain valve, drain valves
A device that puts alcohol into the air system to prevent ice.
Ice inside the valves can stop the brakes working. In cold weather you check the alcohol level every day. You still have to drain the tanks.
On the test
Questions ask why some systems have an alcohol evaporator, what to do with it in cold weather, and what happens if you neglect it.
Appears in 4 test questions
See 4 questions on the Air Brakes testExample: On a January morning you top up the alcohol evaporator before you drain the tanks and leave the yard.
A valve that lets air out when tank pressure gets too high.
It protects the tank and the rest of the system and is usually set to open at 150 psi. If it ever opens, something is wrong and a mechanic needs to look.
Example: You hear air hissing from the first tank while parked; the safety valve has opened, so you report the fault.
Also called: safety relief valve, pressure relief valve
The dashboard gauge that shows the air pressure in the tanks.
Every air-brake vehicle has one. A dual system has two gauges, or one gauge with two needles, one for each half of the system.
On the test
Questions ask what the supply pressure gauge tells you and which gauges you watch while air pressure builds before driving.
Appears in 13 test questions
See 3 questions on the Air Brakes testDoes not mean: The supply pressure gauge shows tank pressure; the application pressure gauge shows how hard you are braking.
Example: You watch both needles climb to normal pressure before you release the parking brake and move off.
Also called: supply pressure gauge
application pressure gauge
A gauge that shows the air pressure you apply to the brakes.
Not every vehicle has one. If you need more pressure to hold the same speed, the brakes are fading or are out of adjustment.
Does not mean: It is not the supply pressure gauge, which shows how much air is stored in the tanks.
Example: Going down a long grade you see the application pressure creeping up, so you slow down and drop a gear.
A warning that comes on when tank air pressure drops too low.
It comes on well before the spring brakes apply on their own. When it comes on, stop safely while you still have air to brake with.
On the test
Questions ask which vehicles need the warning, what pressure triggers it, how to test it, and what to do when it comes on.
Appears in 8 test questions
See 8 questions on the Air Brakes testDiffers by state: Most state handbooks say the warning must come on before tank pressure falls below 55 psi; Texas prints 55 psi. About thirteen handbooks print 60 psi: AZ, DC, GA, LA, MI, NC, ND, NJ, RI, SC, WA, WI and WY. Our New York handbook also prints 60 psi. California alone prints a range, between 55 and 75 psi. Ohio, Maine and Georgia contradict themselves: one line says 55 and another says 60. This is a difference between handbook editions, not between laws. Federal rule 49 CFR 393.51 requires the warning at not less than 55 psi, so both printings comply. Older vehicles may instead warn at one half of the governor cut-out pressure. Large buses commonly warn at 80-85 psi.
Example: The red light and buzzer come on at highway speed; you pull onto the shoulder while the service brakes still work.
Also called: low air warning, low-pressure warning, low air pressure warning signal
A mechanical arm that drops into view when air pressure is too low.
It is an older type of low air pressure warning. An automatic wig wag rises again when pressure returns; a manual one you reset by hand.
Differs by state: The wig wag drops at the same pressure as the low air pressure warning, so it follows the same handbook split. Most states print 55 psi and about thirteen print 60 psi. Ohio prints 55 for the warning light but 60 for the wig wag. California prints a 55-75 psi range.
Example: In an older truck a metal flag swings down in front of you; air pressure has dropped and you must stop.
Diagram coming
The foot control that sends air to the brakes.
Pushing harder sends more air pressure. Releasing lets air out of the system, so the compressor must replace it. Pumping the pedal wastes air.
Does not mean: Foot valve and treadle valve are other names for the same pedal, not separate controls.
Example: You press the brake pedal firmly once instead of pumping it, so you do not waste air pressure.
Also called: foot brake, treadle valve, foot valve
The unit at each wheel where air pressure moves the pushrod.
Air comes in when you press the pedal. The pressure moves the pushrod, which moves the slack adjuster and turns the S-cam.
On the test
Questions ask what happens inside the chamber when you brake and how it connects to the pushrod and slack adjuster.
Appears in 5 test questions
See 4 questions on the Air Brakes testExample: On the walk-around you check the air hose into each brake chamber for cuts or rubbing.
Diagram coming
Also called: brake chambers
The rod the brake chamber pushes out to move the slack adjuster.
How far the pushrod travels shows brake adjustment. If the slack adjuster moves more than about one inch where the pushrod attaches, the brake needs adjusting.
Example: You pull on the slack adjuster where the pushrod attaches and feel how far it moves.
Diagram coming
Also called: push rod, push rods
The lever between pushrod and S-cam that takes up brake slack.
Automatic slack adjusters adjust themselves but still must be checked. If one moves more than about one inch when you pull hard on it, the brake needs adjusting. Brakes out of adjustment are what roadside inspectors find most often.
On the test
Questions ask how to check slack adjusters, how much movement means they need adjusting, and whether automatic ones may be adjusted by hand.
Appears in 13 test questions
See 10 questions on the Air Brakes testExample: With the wheels chocked and the parking brake off, you tug each slack adjuster to feel for free play.
Diagram coming
Also called: slack adjusters
An S-shaped cam that pushes the brake shoes against the drum.
The S-cam drum brake is the most common foundation brake on heavy vehicles. When you release the pedal, the cam turns back and a spring pulls the shoes away.
Example: You press the pedal; the pushrod turns the camshaft, the S-cam spreads the shoes, and the wheel slows.
Diagram coming
Also called: s-cam brakes, cam brakes, camshaft
The metal drum the brake shoes press against to slow the wheel.
The wheel is bolted to the drum and the brake parts sit inside it. Friction makes heat; too much heat makes the brakes fade.
On the test
Questions ask what the shoes push against, what overheating does to drums, and how long a crack may be before the drum fails inspection.
Appears in 8 test questions
See 7 questions on the Air Brakes testExample: On the walk-around you look at each drum for cracks and for oil or grease on the lining.
Diagram coming
Also called: brake drums
The friction material on the brake shoe that rubs against the drum.
Linings must not be loose, oily or dangerously thin. Heat can change the lining and reduce its friction; that is brake fade.
Differs by state: Most handbooks give no number and say only that linings must not be dangerously thin. California (air-brake section 5.3.2) and Colorado print a minimum of 1/4 inch, which is also the federal out-of-service figure. The divergence check covered 20 handbooks, not all states.
Example: You see grease soaked into a lining during inspection, so you report it before the trip.
Also called: brake linings, linings
Brakes that squeeze pads against a spinning disc to slow the wheel.
In air disc brakes a power screw replaces the S-cam and clamps the disc like a big C-clamp. They are less common than S-cam drum brakes on heavy trucks.
On the test
Disc brakes appear mostly as an answer option; questions ask how they differ from S-cam drum brakes and which brake types are common.
Appears in 6 test questions
See 2 questions on the Air Brakes testExample: Your new tractor has air disc brakes, so the walk-around shows a rotor and caliper instead of a drum.
Diagram coming
Also called: disc brake
Brakes applied by powerful springs and held off by air pressure.
While you drive, air holds the springs back. If pressure leaks down to about 20 to 45 psi, the springs put the brakes on by themselves. Spring brakes serve as parking and emergency brakes.
On the test
Questions ask what spring brakes are, what holds them off, at what pressure they apply, and how to check they come on automatically.
Appears in 37 test questions
See 14 questions on the Air Brakes testDoes not mean: Spring brakes are not a separate fourth system; they are how the parking and emergency brakes are applied.
Example: You pull the yellow knob to park; air leaves the chambers and the springs lock the brakes on.
Diagram coming
Also called: spring brake
The brake that holds the vehicle still when parked.
On an air-brake truck you pull the yellow diamond knob to set it and push it in to release. Air can leak away, so the parking brake must hold by mechanical force.
Example: After a long downgrade your brakes are very hot, so you chock the wheels and let them cool before setting the parking brake.
Also called: parking brakes, parking brake control
The brake system that stops the vehicle if the service brakes fail.
It uses parts of the service and parking systems. On most trucks the spring brakes do this job. On a trailer, losing air in the emergency line applies them.
On the test
Questions ask what applies the trailer emergency brakes, what makes them come on suddenly, and what must hold them on.
Appears in 28 test questions
See 11 questions on the Air Brakes testDoes not mean: Not a car-style hand lever; on a truck the spring brakes do this job.
Example: A trailer breaks away on the highway; the emergency line tears, and the trailer's emergency brakes lock on.
Also called: emergency brakes
The brakes you use with the pedal during normal driving.
Pressing the pedal sends air to every wheel, including the trailer. You test them by moving slowly and braking firmly, feeling for pulling or delay.
On the test
Questions ask what the service brake system uses, how to test it at low speed, and how it differs from parking and emergency brakes.
Appears in 31 test questions
See 8 questions on the Air Brakes testExample: At about walking speed you brake firmly and notice the truck pulls left, so you report it.
Also called: service brakes
Two separate air brake systems that share one set of controls.
One system usually works the rear brakes and the other the front, each with its own tanks and lines. Before driving, wait until both halves show at least 100 psi. If one loses pressure you still have some braking, but a longer stop.
On the test
Questions ask what a dual system is, what pressure both halves need before driving, and what to do if one half shows low pressure.
Appears in 8 test questions
See 8 questions on the Air Brakes testDiffers by state: The warning light and buzzer should shut off above a value set by the manufacturer. Most handbooks say that value must be greater than 55 psi. About fourteen say greater than 60 psi, including Maine, whose warning sentence says 55. Our New York handbook also prints 60. It follows the same edition split as the low air pressure warning.
Example: One needle on the gauge sits low while the other is normal; you stop and have the system fixed.
Also called: dual air system, dual air brakes
The delay before air brakes act after you press the pedal.
Air needs time to travel through the lines to the brake chambers. Hydraulic brakes act at once; air brakes take about half a second or more. At 55 mph that delay adds about 32 feet to your stop.
Example: At highway speed you press the pedal and the truck keeps rolling for a moment before the brakes bite.
Loss of braking power because the brakes have overheated.
Heat changes the lining and expands the drum, so the shoes grip less. Fade comes from using the brakes too much instead of the engine on a downgrade. Brakes out of adjustment make the others overheat.
Example: Halfway down a mountain grade the pedal needs more and more pressure to hold speed; the brakes are fading.
Braking fully, releasing when the wheels lock, then braking fully again.
Wait until the wheels start rolling again before you press again, or the vehicle will not straighten out. Vehicles with an anti-lock braking system (ABS) are braked differently: keep firm pressure and let ABS work.
Example: A car pulls out ahead on a wet road; you brake hard, feel the wheels lock, release, and press again.
Braking as hard as possible without locking the wheels.
Keep steering movements very small while you do it. If the wheels lock or you must steer more, release the brakes and apply them again.
Example: Traffic stops suddenly ahead; you squeeze the brakes to just short of lockup and hold the wheel steady.
Stopping as quickly as possible while keeping the vehicle straight and steerable.
Hitting the brakes is the right response only if you have room to stop and brake correctly. Use controlled braking or stab braking. With ABS, brake hard and steer.
Does not mean: Emergency braking is a driving technique; the emergency brake is the system that works when the service brakes fail.
Example: A child runs into the road; you brake hard, stay in your lane, and keep the wheels from locking.
A computer system that keeps the wheels from locking during hard braking.
ABS senses a wheel about to lock and lowers brake pressure at that wheel. It does not make you stop shorter; it helps you keep steering control. A yellow lamp shows a fault.
Does not mean: ABS does not add braking power and does not necessarily shorten stopping distance; it only prevents wheel lockup.
Example: On an icy ramp you brake hard; ABS pulses the brakes and you steer around the stalled car.
Also called: ABS, antilock braking, anti-lock brakes, antilock brakes
A device that slows the vehicle without using the wheel brakes.
There are exhaust, engine, hydraulic and electric retarders. They work on the drive wheels when you lift off the accelerator. Turn them off on wet, icy or snowy roads because the drive wheels can skid.
Example: Rain starts on the descent, so you switch the retarder off to keep the drive wheels from skidding.
Also called: retarders
A retarder that uses the engine itself to slow the vehicle.
Drivers often call it a Jake brake. Be in a low gear before a long downgrade so the engine does most of the slowing. Like any retarder, it acts on the drive wheels only, so switch it off on slippery roads.
Does not mean: The engine's natural braking effect in a low gear is not a fitted engine brake, though both help on downgrades.
Example: You downshift before the grade so the engine brake can hold your speed with little use of the pedal.
Also called: jake brake, engine retarder
A valve that saves the tractor's air if the trailer breaks away.
The trailer air supply knob in the cab opens and closes it. It also closes by itself when pressure drops to about 20 to 45 psi. When it closes, the trailer emergency brakes come on.
Example: During the air brake test you fan the pedal until the red knob pops out; the tractor protection valve has closed.
trailer air supply control
The red knob that sends air to the trailer or shuts it off.
Push it in to supply the trailer with air. Pull it out to cut the air and set the trailer emergency brakes. It pops out on its own when pressure drops into the 20 to 45 psi range.
Does not mean: Out means the trailer brakes are locked and in means air is supplied, not the other way round.
Example: Before you uncouple, you pull the red knob out so the trailer brakes lock before you back off.
Diagram coming
Also called: trailer air supply valve, trailer air supply
A hand lever in the cab that applies only the trailer brakes.
Use it to test the trailer brakes, not while driving: braking the trailer alone can make it skid. Never park with it because the air can leak away.
Does not mean: It is not the trailer air supply knob; the hand valve works the trailer service brakes, the knob the emergency brakes.
Example: After coupling you pull the hand valve and listen for the trailer brakes applying before you drive off.
Also called: trolley valve, trailer brake hand valve
The air line that tells the trailer brakes how hard to apply.
Its pressure follows the brake pedal or the hand valve. It is usually blue. A leak in it shows up only when you try to brake.
Example: You connect the blue service line to the trailer's service glad hand and check that the seal holds.
Diagram coming
Also called: control line
The air line that supplies the trailer and controls its emergency brakes.
It is usually red and is also called the supply line. If it loses pressure, the trailer emergency brakes come on and the tractor protection valve closes.
Example: A red hose tears as the trailer breaks away; the trailer's emergency brakes lock and the tractor keeps its air.
Diagram coming
Also called: supply line
The couplers that join the tractor's air lines to the trailer's.
Each one has a rubber seal; clean it before connecting. Press the two seals together at a right angle and turn to lock. With no trailer attached, park them on dummy couplers or lock them together.
Example: You wipe grit off the seals, then twist the glad hands together until they lock.
Diagram coming
Also called: gladhands
How fast air pressure drops when the engine is off.
You test it twice: with the brakes released, and with the pedal held down for a minute. The limits differ for a single vehicle and a combination.
Differs by state: The applied test (pedal held for one minute) is in every handbook. The limit is less than 3 psi per minute for a single vehicle and less than 4 psi for a combination. The static test (brakes released) is less consistent. Most handbooks print 2 psi single and 3 psi combination. Colorado and Wisconsin omit the static test entirely. California adds a third tier of 5 psi for a combination of three or more vehicles.
Example: Engine off and brakes released, you watch the gauge for a minute to see how much pressure you lose.
Also called: air leak test, leakage rate
Brakes that use fluid pressure, not air, to apply the brakes.
Cars and lighter trucks have them. They work the moment you press the pedal, so there is no brake lag. Test for leaks by pumping and then holding the pedal.
Does not mean: Air over hydraulic brakes count as air brakes for the air brake restriction.
Example: You pump the pedal, hold it firmly, and feel it slowly sink, so you do not drive.
Also called: hydraulic brake
